1. Field of the Invention
The present invention relates to a thermal coupling membrane reactor mainly applied to liquid phase dehydrogenation process of alcohols, in which under the action of selective permeability of the membrane, an endothermic dehydrogenation reaction takes place on one side of the membrane while the exothermic oxidation reaction takes place on the other side, thereby achieving the purposes of in situ heating, improving conversion rate and selectivity of the dehydrogenation reaction and energy saving.
2. Description of Related Art
The preparation of carbonyl-containing compounds from alcohols, as one of the most basic and important reactions, is widely applied in chemical production. Currently, there are two main methods for preparing carbonyl-containing compounds from alcohols, one is catalytic oxidation method in which a carbonyl-containing compound is prepared by the combined action of an oxidant and a catalyst, but this method has many side reactions and may produce toxic and harmful wastes; the other one is catalytic dehydrogenation method in which hydrogen is directly removed under the action of a catalyst, as the following reaction equation (1). Alcohol dehydrogenation is relatively easy to be implemented, with the reaction temperature lower than other dehydrogenation processes, less byproducts and high selectivity, but the conventional dehydrogenation process is generally carried out in the gas phase, with relatively high reaction temperature and relatively large heat consumption, and low equilibrium conversion rate due to reaction equilibrium and temperature limitations. Therefore, the use of liquid phase dehydrogenation method can avoid the above-mentioned disadvantages, and is a preferred method with relatively mild reaction conditions and high selectivity to target products.

In particular, the catalytic dehydrogenation process is implemented by direct dehydrogenation route and transfer dehydrogenation route. As the direct dehydrogenation route generally has a big problem of relatively low equilibrium conversion rate, the reverse hydrogenation reaction may likely take place when the reaction product aldehydes/ketones in the presence of hydrogen. In contrast, the transfer dehydrogenation route avoids such problem. In the transfer dehydrogenation method, a hydrogen acceptor is added into the reaction system, in order to consume the hydrogen generated by the dehydrogenation process. However, this method has strict requirements for catalysts, that is, catalysts are required for both the dehydrogenation process of alcohols and the hydrogenation process of hydrogen acceptors, and also the dehydrogenation and hydrogenation reactions are carried out in the same system, leading to the difficulty in setting process conditions.
It can be seen from the above discussion that a catalyst is required for alcohol dehydrogenation process and the catalyst performance may have a large impact on this reaction system. Based on the difference of states of catalysts, catalysts may be divided into homogeneous catalysts and heterogeneous catalysts. In a homogeneous catalytic system, catalysts are generally active and highly selective, but, the system needs the addition of alkali, organic solvents and other adjuvants, resulting in the corrosion to equipment and the difficulty in separation and recycling of catalysts; while a heterogeneous catalytic system avoids this problem and is environmentally friendly. In a heterogeneous catalytic system, the commonly used catalysts include noble metals of Pd, Pt, Ru and Au loaded on a support. The support including one of metal oxides, molecular sieves, carbon materials and organic polymers; or oxides of non-noble metals Cu, Mn, Ni, Co, Cr and V.
The conventional alcohol dehydrogenation process is mainly carried out in a fixed bed or fluidized bed reactor, but the generation of product hydrogen may limit forward progress of this reversible reaction. In recent years, membrane reactors are increasingly and widely used in the dehydrogenation process, and the so-called membrane reactors combines two separate processes of reaction and membrane separation, which achieves highly efficient reaction and in situ separation at the same time, integrating reaction and separation in one step. Membrane reactors, which are firstly applied to the biological reaction process having mild conditions, usually utilize organic membranes, the occurrence of inorganic membranes makes the commercial application of membrane reactors in petrochemical process possible.
Membranes used in alcohol dehydrogenation membrane reactors should be selectively permeable to hydrogen, and are divided into dense membranes and porous membranes based on the difference of materials. Dense membranes are mainly made of some noble metals, such as palladium and palladium alloys, which have almost 100% selectivity to hydrogen and usually used in production of phenols by benzene hydrogenation, hydrogenation of nitrites, dehydrogenation of hydrocarbons, steam reforming and other hydrogenation or dehydrogenation reactions, preferably in gas phase reactions, and also have relatively low permeability to hydrogen, particularly under relatively low reaction temperature conditions. Porous membranes are mainly made of silica, molecular sieves, carbon and ceramics, may have highly selective permeability to hydrogen under strict control of pore size, and are applicable to both gas phase and liquid phase reactions.
Although membrane reactors are currently applied to dehydrogenation process, they are mostly often used in dehydrogenation of hydrocarbons, steam reforming, water-gas shift reaction and other gas phase dehydrogenation processes. However, there are a few reports on alcohol liquid phase dehydrogenation. Also, in most of the cases reported, hydrogen generated by reaction permeates from the inner side to the outer side of a membrane, and then is blown off by a purging gas or vacuumized to increase the penetration rate, resulting in extra energy consumption and need of handling measures. However, there are a few reports on in situ utilization of hydrogen.
Patent application CN1164523A discloses gas phase catalytic dehydrogenation using a Pb-ceramic composite membrane reactor in which oxidation reaction of hydrogen and oxygen is carried out on one side of a permeable chamber. Although this process utilizes the hydrogen-oxygen reaction to provide heat for the dehydrogenation process, the Pb membrane serves as a catalyst for the reactions on the two sides and also serves for membrane separation, leading to high consumption of Pb membrane, which results in high cost and low permeability, and also, the difficulty in molding the Pb membrane causes difficult match between permeation rate and reaction rate, resulting in poor effect. The reaction rate on oxidation reaction side and the heating amount supplied at each point of dehydrogenation reaction are beyond the control. Further, this patent application is implemented by single tube reaction, resulting in low production efficiency in practical application.
On this basis, the patent application CN1189483A utilizes catalysts filled in the reaction chambers on the two sides of the membrane reactor, for gas phase catalytic dehydrogenation reaction and hydrogenation coupling reaction respectively, and by rational control of the ratio of catalyst usage amount to membrane area, the reaction rate and permeation rate are matched. However, in general, the temperature of dehydrogenation reaction is far higher than that of hydrogenation reaction, as a result, the matching of the temperature of dehydrogenation reaction and the temperature of hydrogenation reaction is a big challenge, and also, hydrogen permeated from dehydrogenation reaction side has a low pressure, the hydrogenation reaction is generally carried out under a relatively high pressure, as a result, the reaction on hydrogenation reaction side is hard to take place.
Overall, although the use of membrane reactors in dehydrogenation reactions has become more common in recent years, they are preferably used for gas phase dehydrogenation reactions using palladium membranes or palladium alloy membranes, and due to relatively low permeability to hydrogen, high cost and difficulty in molding, such membranes are difficult to use in liquid phase reactions; also, in most cases, hydrogen generated by dehydrogenation reactions is blown off by a purging gas or vacuumized rather than fully utilized. Even if treating hydrogen with hydrogenation reaction to implement thermal coupling, the patent application CN1189483A does not consider the dismatch of reaction conditions for hydrogenation and dehydrogenation reactions in industrial applicability, and barely discusses the control of thermal coupling process; furthermore, in terms of industrial applicability, in the case of using the recently reported single tube for dehydrogenation reactions, it is difficult to improve the yield, thereby affecting the benefits of plants.